The term "Metallocene" as used herein refers to a derivative of cyclopentadienylidene which is a metal derivative containing at least one cyclopentadienyl component which is bonded to a transition metal. The transition metal is selected from Groups IVB, VB, and VIB, preferably IVB and VIB. Examples include titanium, zirconium, hafnium, chromium, and vanadium. A number of metallocenes have been found to be useful for the polymerization of olefins. Generally, the more preferred catalysts are metallocenes of Zr, Hf, or Ti.
Generally, in order to obtain the highest activity from metallocene catalysts, it has been necessary to use them with an organoaluminoxane cocatalyst, such as methylaluminoxane. This resulting catalyst system is generally referred to as a homogenous catalyst system since at least part of the metallocene or the organoaluminoxane is in solution in the polymerization media. These homogenous catalyst systems have the disadvantage that when they are used under slurry polymerization conditions, they produce polymer which sticks to reactor walls during the polymerization method and/or polymer having small particle size and low bulk density which limits the commercial utility.
Some attempts to overcome the disadvantages of the homogenous metallocene catalyst systems are disclosed in U.S. Pat. Nos. 5,240,894, 4,871,705; and 5,106,804. Typically, these procedures have involved the prepolymerization of the metallocene aluminoxane catalyst system either in the presence of or in the absence of a support. An evaluation of these techniques has revealed that there is still room for improvement, particularly when the catalyst is one which is to be used in a slurry type polymerization where the object is to produce a slurry of insoluble particles of the end product polymer rather than a solution of polymer which could result in fouling of the reactor. In the operation of a slurry polymerization in a continuous loop reactor it is extremely important for efficient operations to limit polymer fouling of the internal surfaces of the reactor. The term "fouling" as used herein refers to polymer buildup on the surfaces inside the reactor.
Slurry polymerization conducted in a loop reactor (also know as "slurry process") has been generally regarded as being excellent for producing high density resins, but also as being incapable of efficiently producing low density resins, due to, in part, the swelling of the polymer during its production. Using butene as a comonomer helps to produce low density resins, but such resins have poor properties. Thus, slurry polymerization in a loop reactor has been generally omitted from the market for low density resins.
Using metallocene catalysts, under slurry polymerization conditions, and in a loop reactor has presented problems. In general, metallocene catalysts feed very poorly to the reactor. They also have a tendency to produce polymer fines. They also have a tendency of causing irreversible plate-out on the reactor walls leading to loss of heat transfer and eventual reactor shut-down. This plate-out is quite difficult to clean off the reactor walls. The polymerization activity of these metallocene catalysts have often been lower than other catalysts used. Additionally, their activity can be erratic, and can vary with the density of resin produced. Runaways and petering out (the opposite problem) are both quite common with metallocene catalysts. Worst, the catalyst has a bad tendency to produce hard large chunks of polymer of around 0.5 cm diameter, or larger, which causes problems with polymer recovery. These catalysts also have an unusual characteristic of gassing off the reactor (i.e., forming bubbles) even at ethylene levels where this is not supposed to happen based on previous experience with other catalysts. They are extremely sensitive to hydrogen because a small amount can send the melt index off target very easily.
Therefore, the inventors provide this invention to help solve these problems.